1. Field of the Invention
The present invention relates to a ball screw with ball retainer cage equipped between the screw bolt and the nut body so as to isolate two adjacent rolling balls and retain the ball therein even they roll out of the track encircled by the nut body.
2. Description of the Prior Art
A conventional ball screw assembly comprises a screwbolt, a nut unit, and a plurality of rolling balls interposed between the screw bolt and the nut unit. The outer surface of the screw bolt is formed into an arcuate groove spirally encircling thereof. The nut unit is composed of a nut body and accessories for ball circulation. The nut body is formed into a hollow structure to be sleeved over the screw bolt, and the inner surface of the nut body is also formed into a spiral arcuate groove corresponding to that formed on the screw bolt such that the nut assembly is able to displace linearly along longitudinal direction of the screw bolt by mutual rotation of the two components with the aid of the rolling balls rolling therebetween. Although such a movement of the nut assembly is not limited by length of the screw bolt, it is subjected to a large resistance by a frictional force between the rolling balls. This frictional force is not negligible owing to the fact that the rolling balls advance pushing and squeezing along the spiral grooves on which they are unsmoothly contacted. As a result, the durability of the ball screw assembly will be severely affected especially by large instantaneous load variation including abrupt change of rolling direction.
FIG. 10 is a view of an improved ball screw disclosed by U.S. Pat. No. 4,542,661, and FIG. 11 is a cross sectional view cut along line 11xe2x80x9411 of FIG.10. As shown in the two drawings, spiral grooves 11 and 21 are formed on the outer surface of the screw bolt 1 and the inner surface of the nut body 2 respectively. A flange 22 is provided for the nut body 2 for coupling with another device. It is seen that a ball retainer cage 4 with a plurally of retainer holes is provided for isolating adjacent balls so as to prevent mutual collision. However, with this structure, there is no way to prevent balls from dropping down from the groove 11 of the screw bolt 1 except in the region where they are surrounded by the groove 21 of the nut body 2. In the construction illustrated in FIG. 10, a sleeve 5 is enclosed over the nut body 2 to prevent balls 3 from dropping down. The length of the sleeve 5 is estimated to be about half as long as that of the nut body""s stroke. As the sleeve 5 is fixedly secured to the nut body 2, it moves together with the nut body 2. As long as the nut body 2 advances to reach the ultimate point, the sleeve 5 is unavoidably projected ahead of the ultimate point to interfere with a bearing base (not shown) of the ball screw assembly 1. To avoid the aforesaid inconvenience, the distance between the bearing base and the ultimate point of the stroke must be at least 5 times longer than the length of the sleeve 5. Thus, the overall size of the ball screw assembly becomes too bulky, and fabrication cost is exhorbitantly increased.
It is an object of the present invention to provide a ball screw with ball retainer cage which can keep rolling balls working with out squeezing one another and save installation of the protective sleeve as that is done in the above cited case.
It is another object of the present invention to provide a ball screw with ball retainer cage which can prevent the rolling balls from dropping off the retainer cage when the retainer cage is disintegrated from the screw bolt and nut body at the same time.
It is one more object of the present invention to provide a ball screw with ball retainer cage which can shorten the length of the ball screw assembly.
It is still one more object of the present invention to provide a ball screw with ball retainer cage which can be standardized to facilitate mass production with a reduced cost by die casting.
To achieve these and other objects mentioned above, a rolling ball retainer cage is formed between the screw bolt and the nut body with a plurality of retainer holes therein. Each rolling ball occupies one retainer hole so that the two adjacent balls are kept isolated. An extended portion is formed at the outer side of the retainer cage along a spiral groove of the nut body, a protrusion is further formed from both sides of the extended portion at each retainer hole to prevent the rolling ball therein from dropping off outwardly, also another extended portion is formed at the inner side of the retainer cage along a spiral groove of the screw bolt, and a protrusion is further formed from both sides of the extended portion at each retainer hole to prevent said rolling balls therein from dropping off inwardly.
For linking the adjacent rolling ball retainer cages, a joint hole is provide at both ends of the retainer cage so as to connect two terminals thereof with a connecting belt.
For facilitating mass production with a reduced cost, the ball retainer cage is fabricated by extrusion after process of die casting.